Introduction to the Laboratory
The Nottingham Tree-ring Dating Laboratory has held a leading role not only in the initial theoretical development of dendrochronology in Great Britain but in its promotion and practical application as well. Having originally been established over 30 years ago as a joint research programme between the Departments of Mathematics and Archaeology at the University of Nottingham, it is now an independent consultancy. Apart from general commercial undertakings, the Laboratory has also been involved in several major research projects on buildings, the environment and ecology, and, most recently, climate change. The Laboratory maintains academic links with a number of University units, is involved with public lecture programmes, and provides advice and training to other dendrochronologists. The Nottingham Laboratory undertakes work not only for major conservation authorities such as English Heritage and The National Trust, but also for archaeological units, local authorities, architects, developers and builders, estate managers, local history and heritage groups as well as many private individuals. Enquiries are strongly encouraged and are welcomed with enthusiasm and interest.
Introduction to Tree-Ring Dating
Tree-ring dating relies on a few simple, but quite fundamental, principles. Firstly, as is commonly known, trees (particularly oak trees, the most frequently used building timber in England) grow by adding one, and only one, growth-ring to their circumference each, and every, year. Each new annual growth-ring is added to the outside of the previous year's growth just below the bark. The width of this annual growth-ring is largely, though not exclusively, determined by the weather conditions during the growth period (roughly March - September). In general, good conditions produce wider rings and poor conditions produce narrower rings. Thus, over the lifetime of a tree, the annual growth-rings display a climatically influenced pattern. Furthermore, and importantly, all trees growing in the same area at the same time will be influenced by the same growing conditions and the annual growth-rings of all of them will respond in a similar, though not identical, way.
Secondly, because the weather over any number of consecutive years is unique, so too is the growth-ring pattern of the tree. The pattern of a short period of growth, 20, 30 or even 40 consecutive years, might conceivably be repeated two or even three times in the last one thousand years. A short pattern might also be repeated at different time periods in different parts of the country because of differences in regional micro-climates. It is less likely, however, that such problems would occur with the pattern of a longer period of growth, that is, anything in excess of 54 years or so. In essence, a short period of growth, anything less than 54 rings, is not reliable, and the longer the period of time under comparison the better.
The third principle of tree-ring dating is that, until the early- to mid-nineteenth century, builders of timber-framed houses usually obtained all the wood needed for a given structure by felling the necessary trees in a single operation from one patch of woodland, or from closely adjacent woods. Furthermore, and contrary to popular belief, the timber was used "green" and without seasoning, and there was very little long-term storage as in timber-yards of today. This fact has been well established from a number of studies where tree-ring dating has been undertaken in conjunction with documentary studies. Thus, establishing the felling date for a group of timbers gives a very precise indication of the date of their use in a building.
Tree-ring dating relies on obtaining the growth pattern of trees from sample timbers of unknown date by measuring the width of the annual growth-rings. This is done to a tolerance of 1/100 of a millimetre. The growth patterns of these samples of unknown date are then compared with a series of reference patterns or chronologies, the date of each ring of which is known. When the growth-ring sequence of a sample cross-matches repeatedly at the same date span against a series of different relevant reference chronologies the sample can be said to be dated. The degree of cross-matching, that is the measure of similarity between sample and reference, is denoted by a "t-value"; the higher the value the greater the similarity. The greater the similarity the greater is the probability that the patterns of samples and references have been produced by growing under the same conditions at the same time. The statistically accepted fully reliable minimum t-value is 3.5.
However, rather than attempt to date each sample individually it is usual to first compare all the samples from a single building, or phase of a building, with one another, and attempt to cross-match each one with all the others from the same phase or building. When samples from the same phase do cross-match with each other they are combined at their matching positions to form what is known as a "site chronology". As with any set of data, this has the effect of reducing the anomalies of any one individual (brought about in the case of tree-rings by some non-climatic influence) and enhances the overall climatic signal. As stated above, it is the climate that gives the growth pattern its distinctive pattern. The greater the number of samples in a site chronology the greater is the climatic signal of the group and the weaker is the non-climatic input of any one individual.
Furthermore, combining samples in this way to make a site chronology usually has the effect of increasing the time-span that is under comparison. As also mentioned above, the longer the period of growth under consideration, the greater the certainty of the cross-match. Any site chronology with less than about 55 rings is generally too short for reliable dating.
Having obtained a date for the site chronology as a whole, the date spans of the constituent individual samples can then be found, and from this the felling date of the trees represented may be calculated. Where a sample retains complete sapwood, that is, it has the last or outermost ring produced by the tree before it was cut, the last measured ring date is the felling date of the tree. Depending on the date of the last sapwood ring on the other dated samples, and the amount of sapwood they have, the felling date of the other timbers represent can then be deduced.
Where the sapwood is not complete it is necessary to estimate the likely felling date of the tree. Such an estimate can be made with a high degree of reliability because oak trees generally have between 15 to 40 sapwood rings. For example, if a sample with, say, 12 sapwood rings has a last sapwood ring date of 1400, it is 95% certain that the tree represented was felled sometime between 1403 (1400+3 sapwood rings (12+3=15)) and 1428 (1400+28 sapwood rings (12+28=40)).
Given that in a timber-framed building the trees required for each phase are almost certain to have been cut in a single felling operation especially for that building, it is usual to calculate the average date of the heartwood/sapwood boundary of all the dated samples from each phase of a building and add 15 to 40 rings to get the overall likely felling date of the group.
Thus, as a dating method, dendrochronology is relatively simple. There are in reality, however, several possible difficulties. The most common problem is with timbers having insufficient rings, and this can be found even in quite large beams (conversely, large numbers of rings can be found in surprisingly small timbers). There can also be problems with the type of timber. Currently, while elm, chestnut and pine are sometimes found in buildings, it is only oak that can be reliably dated. Even when the right timber with the right number of rings is found some care has to be taken to establish whether the beam is original and integral to the structure or is an older piece reused or a repair or replacement beam. Finally, even buildings that provide seemingly satisfactory samples do sometimes not date, and no-one is quite sure why. It would seem that either the trees used have come from an area/time for which no reference material is available, ie, they are from some climatic niche, or all the trees used have some growth problem which has affected their ring patterns - perhaps they have all been pollarded or all had their leaves stripped. It thus takes some skill on the part of the dendrochronologist to select the right timbers and to core in such a way as to obtain the best samples for dating.